1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device capable of realizing a bright display.
2. Description of the Related Art
A liquid crystal display device is a non-light-emitting device modulating external light, which is characterized by consuming less electric power and being thin and light-weight. Because of the characteristics, a liquid crystal display device is applicable to a flat panel display and has been widely utilized as an information display device such as a clock, a scientific calculator, a computer terminal, a word processor, and a TV receiver.
As is represented by the buzzword "advanced information age", recently, the collection and selection of information demanded by each person increases along with the increase in the amount of information. Under such circumstances, a portable information terminal for personal use has been recognized and actively developed for its realization.
In the portable information terminal, an information display device as a man-machine interface plays an important role and is placed as a key device. The information display device is required of being capable of displaying a large amount of information, being thin and light-weight, being excellent in visual recognition, and consuming less electric power. A liquid crystal display device has been expected as a device satisfying these characteristics and has been actively developed.
Various methods are used for performing a display using liquid crystal. A representative method uses twisted nematic (TN) liquid crystal. According to this method, a liquid crystal cell including two substrates and liquid crystal inserted therebetween is used, in which the orientation of the liquid crystal molecules from one substrate to the other is twisted by 90.degree.. When being aligned in this manner, liquid crystal molecules exhibit optical rotation which is eliminated by changing the orientation of the liquid crystal molecules with a voltage or the like. This allows the amount of transmitted light to be varied and modulates light intensity, whereby a display is performed.
The TN method uses a low voltage, consumes less electric power, and realizes a gray-scale display with a satisfactory contrast. For these reasons, the TN method is widely used for a liquid crystal display among various operation modes.
The TN method has outstanding characteristics as described above. However, the threshold characteristics of TN liquid crystal are not so steep. Therefore, in order to perform a large capacity display, active elements such as thin film transistors (TFTs) are generally used together with TN liquid crystal. On the other hand, a super twisted nematic (STN) method using liquid crystal which is twisted by more than 90.degree. is proposed. This method uses optical rotation and birefringence of STN liquid crystal which has a steep threshold property. Therefore, this method is suitable for multiplex driving and is capable of performing a large capacity display without using active elements. This method cannot perform a monochromic display since it uses optical rotation and birefringence of liquid crystal. However, an achromatic display is achieved by optical phase compensation. Thus, this method is used for a display device of a word processor and a portable computer.
Furthermore, a method using ferroelectric liquid crystal and birefringence thereof is proposed. This method is characterized by rapid response and a memory property.
There is also a display method using liquid crystal dispersed in a polymer matrix. This method utilizes a scattering effect brought about by the inconsistency in a refractive index between the polymer and the liquid crystal. This method enables a display to be performed without using polarizing plates, so that a bright display with a large viewing angle can be realized.
In addition, there is a guest-host (GH) mode. This mode uses liquid crystal with a pleochroic dye mixed therein. According to this mode, a display is performed by utilizing absorption anisotropy of dye molecules exhibited by the changes in the orientation thereof caused by the changes in the orientation of liquid crystal molecules. Depending upon the orientation state of liquid crystal molecules and the combination of optical elements, various GH display modes are proposed (Mol. Crysta. Liq. Cryst. vol. 63, p. 19, 1981).
A Heilmeier-type GH display mode using uniformly aligned liquid crystal molecules and polarizing plates is capable of performing a gray-scale display with a satisfactory contrast (Appl. Phys. Lett. vol. 13, p. 91, 1968).
There is also a GH display mode realizing a display with a satisfactory contrast without using polarizing plates (SID Symposium Digest p. 96, 1977). According to this mode, a reflective display is realized by combining a quarter-wave plate with a liquid crystal layer in which liquid crystal molecules are uniformly aligned. This mode does not use polarizing plates, so that it enables a gray-scale display with high brightness.
Another GH display mode is proposed (SID Symposium Digest, p. 192, 1980). According to this mode, a display is performed by using two GH liquid crystal cells layered in such a manner that uniformly aligned liquid crystal molecules in each cell are orthogonal to each other. This mode does not use polarizing plates, so that it realizes a gray-scale display with high brightness and a satisfactory contrast.
Furthermore, a phase transition type GH display mode is proposed (J. Appl. Phys. vol. 45, p. 4718, 1974). According to this mode, a liquid crystal layer made of chiral nematic liquid crystal with a dichroic dye mixed therein is inserted between two substrates the surface of which is homeotropically aligned, and a display is performed by using absorption of the dye involved in nematic cholesteric phase transition. This mode does not use polarizing plates, so that it enables a bright display to be performed.
In any of the above-mentioned GH display modes, a viewing angle is large. The GH display mode using a quarter-wave plate, the two-layer type GH display mode, and the phase transition type GH display mode do not use polarizing plates. Therefore, a bright display can be realized.
Still furthermore, a GH display mode, in which liquid crystal molecules are horizontally aligned on the surfaces of substrates and the liquid crystal molecules are twisted from one substrate to the other, is proposed (Japanese Laid-Open Patent Publication No. 59-28130). This mode realizes a large capacity display using simple matrix drive, as well as high brightness.
According to the above publication, in the case where the refractive index anisotropy of liquid crystal molecules is small (.DELTA.n.ltoreq.0.15 in the publication), the light absorption by dye molecules is large. Therefore, it is desired that a device is used without polarizing plates so as to obtain a brighter ON-state. In the case where the refractive index anisotropy of liquid crystal molecules is large (.DELTA.n.gtoreq.0.15 in the publication), it is desired that a single polarizing plate is used so as to increase the light absorption by dye molecules in an OFF-state.
However, the above-mentioned conventional liquid crystal display devices have the following problems.
Regarding the TN and STN methods using polarizing plates, since half of external light is absorbed, light is not efficiently used, making it difficult to obtain a bright display. In the TN method, it is required to provide an active element for each pixel so as to perform a large capacity display. This blocks light, resulting in decrease in utilization efficiency of light. Moreover, a light-blocking layer called a black matrix is provided so as to cover portions other than pixels for the purpose of improving a contrast. This decreases the utilization efficiency of light. In a TN-type large capacity display device using active elements, a traverse electric field between adjacent pixels causes disclination lines to decrease a contrast. In order to solve this problem, disclination lines are covered with a light-blocking layer so as to improve a contrast. However, this, in turn, decreases the utilization efficiency of light.
Because of various reasons as described above, a generally used large capacity display device with TFTs require an external light source such as a backlight so as to obtain a bright display. This enables a satisfactory display to be obtained, while the power consumption by a backlight is larger than that of liquid crystal and the backlight and optical portions thereof are thicker than a liquid crystal display cell. Thus, TN and STN displays cannot realize characteristics such as thinness, light-weight, and low power consumption.
The method utilizing the birefringence of ferroelectric liquid crystal has a problem that a bright display is not obtained because of the use of two polarizing plates and it is difficult to uniformly align ferroelectric liquid crystal molecules.
According to the method utilizing the scattering effect brought about by the inconsistency in a refractive index between a polymer and liquid crystal, polarizing plates are not used, so that a bright display can be obtained. However, it requires a high drive voltage and has an unsatisfactory contrast due to insufficient scattering of light.
According to the method using the combination of uniformly aligned GH liquid crystal and polarizing plates, polarizing plates are used, so that sufficient brightness is not obtained. The reason for using polarizing plates in this mode is to improve a contrast. More specifically, in this mode, polarized light in parallel with an absorption axis of dye molecules is absorbed while polarized light perpendicular to the absorption axis is hardly absorbed. Therefore, light is not sufficiently absorbed with no voltage applied, and hence, a satisfactory contrast is not obtained without polarizing plates.
According to the method for performing a reflective display, using the combination of uniformly aligned GH liquid crystal with a quarter-wave plate, polarizing plates are not used, so that a bright display can be realized. However, the quarter-wave plate satisfies quarter-wave conditions only at a particular wavelength. Therefore, this method is not suitable for a monochromic display.
According to the method for performing a display, using two liquid crystal cells layered in such a manner that the orientation of uniformly aligned liquid crystal molecules in each cell is orthogonal to each other, polarizing plates are not used, so that a gray-scale display with high brightness is obtained. However, it is required to simultaneously drive two liquid crystal cells and it is also difficult to produce a device with such a structure.
In contrast, according to the GH display mode with a structure in which the orientation of liquid crystal molecules is twisted between substrates, the rotation of polarization plane of light does not follow the changes in orientation of liquid crystal molecules. Therefore, light is sufficiently absorbed with no voltage applied, even when polarizing plates are not used. This relatively improves a contrast. In general, as the twist angle becomes larger, the contrast becomes better. However, a voltage required for driving a liquid crystal layer increases with the increase in the twist angle of liquid crystal molecules, and hysteresis is caused in the dependency of light transmittance on an applied voltage. Therefore, a gray-scale display cannot be performed.
A display device using a GH cell is realized when a twist angle of liquid crystal molecules is in the range of n radian to less than 2.pi. radian, as disclosed in Japanese Laid-Open Patent Publication No. 59-28130. A GH cell in which hysteresis is not caused in the dependency of light transmittance on an applied voltage is achieved by increasing a pretilt angle of liquid crystal molecules. However, in this case, a gray-scale display cannot be performed at a twist angle of more than 2.pi. radian. Even at a twist angle of 2.pi. radian or less, there are angles at which the light transmittance decreases under the application of a voltage, and therefore, sufficient brightness cannot be obtained. Furthermore, the above-mentioned publication fails to describe the relationship between the twist angle of liquid crystal molecules and the brightness in an ON-state.